1. Field of the Invention
The present invention generally relates to a protection apparatus and a method for fabricating the same, and more particularly, to a thermal protection apparatus and a method for protecting an electronic system using the same.
2. Description of Related Art
FIG. 1 illustrates a conventional thermal shutdown circuit, which is disclosed in U.S. Pat. No. 5,737,170. Referring to FIG. 1, in the conventional thermal shutdown circuit 100, resistors 103 and 104 cooperatively form a voltage divider circuit, for dividing a bias voltage VBG and then outputting the divided voltage to the base of a BJT transistor 105.
Because the turn-on voltage of the BJT transistor has a negative temperature coefficient characteristics, the turn-on voltage (VBE) drops approximately 2 mV for every 1° C. rise in the ambient temperature. Once the ambient temperature rises to a critical temperature, the BJT transistor 105 will be turn on to shutdown the electronic system 120, thereby providing thermal protection to the electronic system 120.
The above action is operated under the precondition of a presumed zero temperature coefficient of the bias voltage VBG. In practice, however, the bias voltage VBG may have positive or negative temperature coefficient characteristics. Therefore, the bias voltage VBG may be subject to a voltage drift as ambient temperature changes, which may cause problems in operation. For example, although the ambient temperature does not reach the critical temperature, the drift of the bias voltage VBG may cause an early turn on of the BJT transistor 105; on the other hand, although the ambient temperature rises beyond the critical temperature, the BJT transistor 105 is not turn on, both conditions can cause damage to the electronic system 120.
In addition, not only the drift of the bias voltage VBG, but also the process of fabricating the BJT transistor 105 can cause the turn-on voltage to be nonlinear with the ambient temperature. That is, as the ambient temperature rises, the turn-on voltage of the BJT transistor 105 may drop less than 2 mV or more than 2 mV for every 1° C. rise in the ambient temperature. Accordingly, the conventional thermal shutdown circuit can not provide an effective thermal protection to the electronic system 120.
Therefore, how to provide a reliable protection apparatus is an important subject of research.